Clinical, pathologic, and genetic studies on two colorectal cancer-prone families have disclosed right-sided colonic flat adenomas and colorectal cancer. Adenomatous polyp counts exceeded those found in hereditary nonpolyposis colorectal cancer (HNPCC) but were fewer than in familial adenomatous polyposis (FAP). Colon cancer occurred at a later age than in HNPCC or FAP and showed right-sided predominance. The older age of patients with colonic cancer, the rightsided predominance of colon cancer, and the paucity of rectal adenomas make FAP unlikely. Vertical transmission of polyps and colon cancer fit the pattern of autosomal dominant inheritance. A characteristic feature of this phenotype is the predominance of flat adenomas. Molecular genetic studies, with careful description of phenotype, should help clarify classification. Cancer 66:909-915,1990.
s RECENTLY as 2 decades ago, the only hereditary A form of colorectal cancer recognized by clinicians and geneticists was familial adenomatous polyposis (FAP),' which accounts for less than 1% of all colorectal cancers. Recent interest in the role of host factors in colorectal cancer has led to a broader view of the subject as evidenced by numerous well-documented families with hereditary nonpolyposis colorectal cancer ( HNPCC).293
Hereditary nonpolyposis colorectal cancer is characterized by an autosomal dominantly inherited predisposition to early onset and multiple colorectal cancers, predominantly in the proximal colon. HNPCC is much more